feat: Vike +data 服务端取数 / SEO Head 移植 / 404 错误页 / 服务端 DOM 模拟

- feedData.ts 聚合首页/板块/帖子详情数据，缺失内容 throw render(404)
- Go 侧 spa_meta/seo 的 head 生成逻辑移植到 src/ssr/head.ts
- _error 页复用站点壳渲染 404，+data 从 abortReason 恢复壳层数据
- linkedom 模拟服务端 DOM，DOMPurify 封装为 SSR 透传的 sanitizeHtml
- 新增 /api/home-boot 聚合端点（handler/home_boot.go）供 SSR 冷启动灌缓存

frontend/src/components/CommentEditor.tsx

@@ -6,7 +6,7 @@ import StarterKit from '@tiptap/starter-kit';
 import Link from '@tiptap/extension-link';
 import Placeholder from '@tiptap/extension-placeholder';
 import Underline from '@tiptap/extension-underline';
-import DOMPurify from 'dompurify';
+import { sanitizeHtml as sanitizeContentHtml } from '../utils/purify';
 import {
   Bold, Italic, Underline as UnderlineIcon, Strikethrough,
   List, ListOrdered, Code, Link as LinkIcon, Image as ImageIcon,
@@ -36,7 +36,7 @@ interface Props {
 }
 
 function sanitizeHtml(html: string): string {
-  return DOMPurify.sanitize(html, POST_CONTENT_PURIFY_CONFIG);
+  return sanitizeContentHtml(html, POST_CONTENT_PURIFY_CONFIG);
 }
 
 function isEditorEmpty(editor: Editor): boolean {